It is frequently desired to produce a first signal at a predetermined frequency, phase synchronized to a second, reference signal which may have a much lower frequency. A PLL (phase locked loop) may typically be used for this purpose, the PLL including a VCO (voltage controlled oscillator), a frequency divider, and a phase detector or comparator. An output signal of the VCO, constituting the first signal, is frequency divided by the frequency divider to the frequency of the reference signal, and the resulting signal is compared in phase with the reference signal by the phase detector to produce a control signal for controlling the VCO.
Whilst such a PLL is effective for relatively low-frequency signals, it necessitates the provision of a frequency divider. For high-frequency signals, for example where the first signal has a frequency greater than 1 GHz for use for example in microwave radio transmission systems, conventional forms of the PLL are inconvenient, making it necessary to provide relatively complicated circuitry for performing phase detection at lower frequencies (e.g. at the transmission system intermediate frequency).
An object of this invention, therefore, is to provide an improved phase detector and apparatus including such a phase detector.